Hermetically sealed electromagnetic relay

ABSTRACT

Sealed relays intended for high-voltage or high-power switching are enclosed in a herrnetically sealed plastic housing or jacket ( 11 ) capable of long-term maintenance of either a high vacuum or a pressurized insulating gas within the relay to suppress contact arcing during switching. The impermeable plastic housing eliminates need for conventional glass or ceramic contact enclosures, and enables use of inexpensive relays ( 71 ) in demanding applications.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation in part of U.S. Provisional PatentApplication No. 60/012,337 filed Feb. 27, 1996.

BACKGROUND OF THE INVENTION

Hermetically sealed electromagnetic relays are used for switching ofhigh electrical currents and/or high voltages, and typically have fixedand movable contacts, and an actuating mechanism supported within ahermetically sealed chamber. To suppress arc formation, and to providelong operating life, air is removed from the sealed chamber byconventional high-vacuum equipment and techniques. In one style ofrelay, the chamber is then sealed so the fixed and movable contactscoact in a high-vacuum environment. In another common style, theevacuated chamber is backfilled (and sometimes pressurized) with aninsulating gas (e.g., sulphur hexafluoride) with good arc-suppressingproperties.

The sealed chamber is conventionally formed by a glass or ceramicenvelope which is fused (glass-to-metal seal) or brazed(ceramic-to-metal seal) to metal components of the relay such asterminal pins and a typically cylindrical or tubular metal base. Thesefused or brazed junctions are specified by Military SpecificationMIL-R-83725 with respect to high-voltage sealed relays.

Properly selected grades of glass or ceramic provide the essentialcharacteristics of low gas permeability, excellent insulating ordielectric qualities, low outgassing, and mechanical strength. Glassenvelopes, however, are handmade by skilled artisans, and are expensiveand subject to breakage, and ceramic envelopes are both expensive topress and metalize, and difficult to procure. It is to the solution ofthese problems that our invention is directed.

Our improvement is directed to the replacement of these glass or ceramicchamber-enclosing envelopes with an inexpensive and easily formedvacuum-tight assembly of plastic and epoxy, or in an alternative form,an envelope made entirely of epoxy. We have established that this typeof plastic/epoxy or epoxy envelope provides an excellent hermetic seal,good dielectric and outgassing characteristics, and a strong,inexpensive sealed relay for switching high currents and/or highvoltage.

Attempts have been made in known designs to use plastic materials inrelays, and U.S. Pat. Nos. 4,039,984, 4,168,480 and 4,880,947 areexamples of the use of epoxy resins as adhesives to secure togetherrelay housing components. Curing of the epoxy to a cross-linkedthermoset state shrinks the joint bond and weakens the seal. Certainother designs (e.g., U.S. Pat. 5,554,963) have used thermoplastic (asopposed to cross-linked thermosetting) polymers, but the resulting relayenvelope is not a true hermetic seal which can maintain either ahigh-vacuum or high-pressure environment.

For purposes of this invention disclosure, a hermetic seal means a sealwhich is sufficiently strong and impermeable to maintain for a long terma high vacuum of 10−⁵ Torr (760 Torr=one atmosphere) or less, and apressure of at least 1.5 atmospheres. In contrast to the prior-artdesigns, the present invention achieves hermetic sealing byencapsulating the relay chamber in a jacket of impermeable epoxy or acomparable thermosetting polymer, the jacket having single-junctionepoxy-to-metal bonds. Shrinkage of the epoxy during polymerization is asignificant advantage in the invention as it provides a strong andreliable single-junction seal.

In one embodiment described below, an unsealed relay is encapsulated ina vacuum chamber, thus eliminating the need for an evacuation tube whichcharacterizes prior relay designs. This same new method can be used tomake pressurized relays which are evacuated, backfilled and encapsulatedwithin a properly equipped chamber.

SUMMARY OF THE INVENTION

This invention is directed to the replacement of glass or ceramiccontact-enclosing housings in sealed relays with an economicalthermosetting-plastic jacket which is impermeable to inflow of air in ahigh-vacuum relay, and to outflow of insulating gas in a backfilled andpressurized relay. Epoxy is a presently preferred material because itforms hermetic seals with impermeable metal components (such asterminals) which must extend through the jacket, and is substantiallyimpermeable to gasses of small molecular size such as hydrogen.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a sealed relay according to theinvention;

FIG. 2 is an enlarged sectional elevation of the relay beforeencapsulation, on line 2—2 of FIG. 5;

FIG. 3 is a reduced sectional elevation of the relay on line 3—3 of FIG.5;

FIG. 4 is a top sectional view of the relay on line 4—4 of FIG. 2;

FIG. 5 is a top view of the assembly shown in FIG. 2;

FIG. 6 is an elevation of a cylindrical assembly which supports terminalpins and fixed/movable contacts of the relay;

FIG. 7 is a sectional elevation on line 7—7 of FIG. 6;

FIG. 8 is a bottom plan view on line 8—8 of FIG. 7;

FIG. 9 is an enlarged elevation of detail shown in the lower-rightcorners of FIGS. 2 and 3;

FIGS. 10A and 10B are respectively a sectional side elevation and a topview of second embodiment of the invention using an open-frame relay ina plastic cup supported in an outer metal cup, the assembly being shownbefore encapsulation;

FIG. 11 shows the assembly of FIGS. 10A and B in a closed chamber havingevacuation, pressurization and encapsulation-material valves; and

FIG. 12 is a view similar to FIG. 11, and showing the relay assemblyfilled with cured encapsulation material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a sealed relay 10 using a plastic and epoxy-sealed envelopeto enclose the fixed and moving contacts of the relay. A primaryexternal sidewall of the relay is formed by a plastic potting cup 11which serves as a mold to hold epoxy material 12 poured into the cup andcured to provide a hermetic seal. Insulated electrical leads 13 extendthrough the epoxy material for connection of fixed and movable contactsto external circuitry. A threaded metal mounting base 14 extends throughthe underside of cup 11, and has a lower end closed by a metal coverplate 15 secured by a nut 16, and through which a pair of actuating-coilleads 17 extend for connection to external circuitry.

The concepts of the invention are useful in many different styles ofhermetically sealed relays (whether of a high-vacuum type, or aback-filled or pressurized type), and will be described in the contextof a double-pole double-throw relay using a conventional and typicalelectromagnetic actuator and fixed and movable contact assemblies. Theinvention is not limited to this specific configuration which isillustrated only by way of example, and is equally applicable to othertypes of sealed relays.

Referring to the sectional elevations of FIGS. 2 and 3, base 14 (made ofa high-permeability magnetic-metal alloy such as C1018 iron) has acylindrical sidewall 18, a central cylindrical pole piece 19, and anannular space 20 between the sidewall and pole piece into which isfitted a conventional actuating coil (not shown). The upper end of space20 is closed by a washer-like disk 22 made of a non-magnetic materialsuch as monel metal, and which is brazed to the sidewall and pole pieceto provide a hermetic seal.

A movable armature 23 is pivotally mounted to the top of the base by ahinge (not shown). A coil spring 25 is seated in an annular space 26between the upper ends of the sidewall and pole piece above disk 22, andurges the armature away from the pole piece when the relay is in anonenergized condition. The armature has an upwardly extending actuatingleg 27 with a slot 28 (FIG. 3) at its upper end. The pole piece has acentral bore 29 extending to an evacuation tube 30 brazed andhermetically sealed to the pole piece, and through which a sealedchamber 31 of the relay can be pumped down to a high vacuum (and, ifdesired, backfilled to a pressure of say three atmospheres with aninsulating gas such as sulphur hexafluoride). Tube 30 is thereafterpinched off and sealed where it extends through an externally threadedboss 32 which receives nut 16.

Sealed chamber 31 is enclosed by base 14 and a hollow assembly 35 asbest seen in FIGS. 6-8. Assembly 35 includes a generally cylindricalplastic sidewall 36, an upper closure cap 37 press fitted into the upperend of the sidewall, and a metal ring 38 press fitted into the lower endof the sidewall and having at its lower end an outwardly extendingflange 39 which is brazed to a metal disk 40 which is in turn brazed toa disk 41 brazed to an inwardly extending annular shoulder 41 in theouter surface of base 14 (FIGS. 2 and 9). These brazed junctionshermetically seal the joined components.

Six metal terminal pins 44 a-f are radially spaced apart, and extendthrough sidewall 36 to form the six terminals of a DPDT switch. Pins 44are fixtured in an injection mold in which plastic sidewall 36 isformed, and are thereby rigidly supported by the sidewall. Pins 44 a-band d-e form fixed contacts of the switch, and pins 44 c and f areconductive posts on which a pair of movable contacts 45 (FIG. 4) aremounted. External leads 13 are secured to the pins by connectors 46secured to the pins.

Each movable contact is Y-shaped in plan view (FIGS. 4 and 8) to definea pair of contact surfaces 48 which are urged against or away from oneof the associated pair of fixed contacts in seesaw fashion when therelay is energized or deenergized. Each movable contact has a pair ofdownwardly extending inner and outer tabs 49 and 50 each having a holeat its upper end so the contact can be fitted over associated pin 44.

A lower hole 51 extends through each inner tab 49 to receive aninsulated rod 52 which couples the movable contacts together. Rod 52 isfitted into slot 28 of armature leg 27 (FIG. 3), and is held captivebetween the movable contacts by a lower end 53 of each outer tab 50.This general style of fixed and movable contact assembly isconventional, and is described in greater detail in, for example, U.S.Pat. No. 3,604,870, the disclosure of which is incorporated herein byreference.

The relay is assembled by placing assembly 35 against base 14 with ringflange 39 against disk 40, and insulated rod 52 engaged in slot 28 ofthe armature leg. With cap 37 removed, proper alignment of the parts cannow be checked by actuating the relay coil, and any necessaryadjustments are made before welding ring flange 39 to disk 40. Cap 37 isthen press fitted into sidewall 36, and an O-ring 55 is fitted into anannular groove 56 in the outer surface of base sidewall 18 beneath disk40 (FIG. 9).

Open-top plastic (Nylon 6/6 is a presently preferred material) pottingcup 11 has a hexagonal sidewall 61 and a bottom wall 62 having a centralcircular opening 63 which receives the threaded lower end of base 14 asshown in FIGS. 2 and 8. Optional mounting tabs 64 (shown in FIGS. 3-5)may be integrally molded with the potting cup if desired. The pottingcup is tightened on base 14 to compress O-ring 55 by temporarilytightening a nut (not shown) on the externally threaded part of the baseagainst the cup.

With the assembly fixtured in an upright position, external leads 13 aresupported to extend vertically from pins 44, and uncured epoxy 12 isthen poured into a space 66 between the exposed outer surfaces ofassembly 35 and base 14, and the inner surface of the potting cup. Theepoxy also covers the top of assembly 35, and fills the potting cup asshown in FIG. 1. After conventional curing of the epoxy, the relay isevacuated (and, if desired, backfilled) through tube 30 which is thensealed by cold-weld pinch off, and the relay coil and associated coverplate 15 are secured in place by nut 16.

The body of encapsulating epoxy 12 forms a hermetic seal around all ofthe components which define sealed chamber 31. More specifically,hermetic seals are formed at the epoxy-to-metal junctions of the epoxywith pins 44 where they emerge from sidewall 36, with connectors 46,with the exposed portions of ring 38, disk 40 and sidewall 18 of thebase. O-ring 55 is not relied on for a hermetic seal, and is insteadused only to prevent leakage of uncured epoxy during the pouring andcuring cycles.

A second embodiment of a sealed relay according to the invention isshown in FIGS. 10-12, and this embodiment uses a simple and inexpensiveopen-frame relay in an open-top housing assembly which is evacuated,encapsulated and backfilled while positioned within a sealed chamber.This manufacturing method eliminates need for an evacuating andbackfilling tubulation, and enables use of an inexpensive relay forhigh-voltage and high-power applications heretofore handled only by moreexpensive high-vacuum or pressurized units of known types as describedin the introductory part of this specification.

Referring to FIGS. 10A and B, a relay assembly 70 is shown prior toencapsulation, and the assembly includes a conventional open-frame relay71 (illustrated as a single-pole single-throw or SPST type, but otherconventional contact configurations are equally useful) secured to andsuspended from a generally rectangular header 72. Elongated metalterminal pins 73 a-d extend through the header, and pins 73 a and b areconnected to a coil 74 of the relay electromagnetic actuator. Pin 73 csupports a fixed contact 75, and pin 73 d is connected to a movablecontact 76 which is pulled against the fixed contact when the relay isenergized. A coil spring 77 urges the movable contact into an openposition in conventional fashion.

Relay 71 is positioned within an open-top plastic cup 79, with theunderside of header 72 supported on short spaced-apart lugs 80 whichextend inwardly from the inner perimeter of a sidewall 81 of cup 79slightly below the top of the cup. The header does not make a snug pressfit within the upper end of the cup, and there is instead an intentionalnarrow gap 82 of say 0.002-0.003 inch between the side edges of theheader and the inner surface of sidewall 81.

Plastic cup 79 is in turn centrally fitted within an open-top metal cup84 having a base 85 against which the plastic cup rests, and an upwardlyextending sidewall 86. The plastic cup is smaller in external dimensionthan the interior of sidewall 86, creating a space or gap 87 between theplastic and metal cups. Sidewall 86 extends higher than the top of theplastic cup, and pins 73 a-d in turn extend higher than the top of themetal cup. An acceptable alternative to metal cup 84 is a similarlyshaped plastic cup having a separate metal plate resting on the cupbottom for bonding with encapsulation material.

The thus-assembled components are next placed in a sealed chamber 89 asshown in FIG. 11. The chamber has an evacuation valve 90 connected to ahigh-vacuum pumping system (not shown) of a conventional type usingmechanical and diffusion pumps. The chamber also has a pressurizationvalve 91 connected to a pressurized source (not shown) of an insulatinggas such as SF₆. The chamber further has a third valve 92 positionedabove cup 84, and connected to a piston-cylinder assembly 93 for holdingand delivering a metered amount of uncured viscous, but fluidencapsulating material 94.

Evacuation valve 90 is then opened, and the high-vacuum pumping systemactuated to withdraw air from the chamber interior to a vacuum which ispreferably at least 10−² to 10−³ Torr if the relay is to be backfilled.Ambient air is simultaneously withdrawn from relay assembly 70 throughgap 82 between header 72 and sidewall 81. Valve 90 is closed when adesired vacuum is achieved.

Open-frame relays are unsuited for long-term vacuum operation due tooutgassing of components such as the relay coil which will eventuallycontaminate and adversely affect a high-vacuum environment. This problemis eliminated by backfilling and pressurizing the chamber andas-yet-unsealed relay assembly with an insulating gas which is admittedby opening pressurization valve 91. The gas flows freely through gap 82to fill and pressurize the interior of the relay assembly.

With the chamber interior stabilized in a high-pressure condition, valve90 is closed, valve 92 is opened, and piston-cylinder assembly 93actuated to deliver at a pressure exceeding that of the pressurizedchamber a metered amount of fluid encapsulating material into metal cup84 to completely fill gap 87 and cup 84 to a level just beneath the topof sidewall 86 as shown in FIG. 12. The encapsulating material is tooviscous to pass through small gap 82, and the backfilled environmentwithin the relay assembly remains undisturbed.

Preferably, chamber 89 is of a conventional type which includes a heatersuch as an induction heater, and heat is applied to the now-encapsulatedrelay assembly to cross link and cure the encapsulating material. Withthe chamber vented to atmosphere, the completed relay assembly isremoved for testing and packaging. In production, many relay assemblieswould be processed in a single loading of the chamber, and the methodsof the invention can also be adapted for use in a continuous productionline.

The optimum environment in which the relay contacts make and break isdependent upon the required performance of the relay. Vacuum (less than10−⁵ Torr) is generally a good environment for high-voltageapplications, but would not be chosen for applications where relaycomponents in the vacuum environment might outgas. There are many gasesthat can be used to improve electrical performance of a relay. Sulfurhexafluoride (SF₆) is a good dielectric gas which at higher pressurewill standoff significantly higher voltages than open air. A relay thatwill standoff 5 kilovolts in open air will standoff 40 kilovolts if itis pressurized with 10 atmospheres of SF₆. Another characteristic of SF₆is that once ionized it becomes an excellent conductor. This makes it agood choice for relays that need to make into a load and keep consistentconduction of current while the load is being discharged. It is not agood gas, however, if that load needs to be interrupted, because the SF₆will tend to continue conduction, and prevent the load from beinginterrupted.

Hydrogen (and hydrogen-nitrogen blends) has been shown to effectivelycool the electrical arc that is created when the electrical contactsmove away from each other while breaking a load. The difficulty withhydrogen is that not only is it the smallest molecule so that it willpropagate through the smallest cracks, but it can also chemicallypropagate through many materials. The design of the present inventionusing cross-linked polymers, unlike other designs, will hold pressurizedhydrogen gas for many years.

There are several kinds of epoxy materials which bond satisfactorilywith metal and, which are impermeable to prevent leakage of air into avacuum relay, or loss of insulating gas in a pressurized relay. Apresently preferred material is commercially available under thetrademark Resinform RF-5407(75% alumina filled) mixed 100:12 by weightwith Resinform RF-24 hardener. Alternative epoxy materials shouldprovide these characteristics:

a. Low gas permeability (less than 10−¹⁰ standard cubic centimeters ofair per second).

b. High dielectric strength (greater than 100 volts per mil).

c. Low outgassing (to maintain a vacuum of 10−⁵ Torr or better).

d. Good mechanical strength.

e. Thermal expansion characteristics reasonably matched to those of themetal with which the epoxy forms a hermetic seal.

There have been described several embodiments of epoxy envelopes forhermetically sealing standard relay designs in a special atmosphere forimproved performance. These envelopes provide significant cost savingsin the manufacture of vacuum or pressurized sealed relays, and haveperformance characteristics at least equivalent to relays of this typeusing glass or ceramic envelopes. The invention is not limited to thespecific relay types described above, and is equally useful with otherswitching devices such as reed-style relays and the like.

We claim:
 1. A sealed electromagnetic relay assembly comprising: a relayhaving a plurality of leads for connection to external circuitry; ahermetically sealed housing assembly enclosing the relay, the housingassembly comprising: a base supporting the relay; the base including anevacuation tube, the evacuation tube being in fluid communication withan interior chamber of the housing assembly, wherein ambient air may beevacuated from the housing assembly to a vacuum within the range ofabout 10−⁵ Torr or less and wherein the housing assembly afterevacuation is backfilled with an insulative gas to a pressure of 1.5atmospheres, a hollow assembly attached to the base, the hollow assemblydefining an interior chamber surrounding the relay; terminal pinsconnected to the relay leads, the pins extending through a wall of thehollow assembly; an upper closure, the upper closure being attached toand closing off the hollow assembly; and an impermeable potting cupsurrounding the sealed housing assembly, the potting cup being adaptedto receive the base at one end and being open at the other end for thereceipt of encapsulating material, wherein the encapsulating materialseals the housing assembly, and the relay leads extending outward fromthe housing assembly, against ambient air intrusion.
 2. The sealedelectromagnectic relay of claim 1, wherein the hollow assembly furthercomprises a hollow cylindrical upper plastic portion which is press fitinto a metal ring.
 3. The sealed electromagnectic relay of claim 2,wherein the base member is of generally circular configuration, the basemember being formed of metal and attached to the hollow assembly bybrazing.
 4. The sealed electromagnectic relay of claim 2, wherein thebase member further includes an o-ring groove and an o-ring disposedwithin the o-ring groove, wherein the base is received within theimpermeable potting cup such that the cup seals against the o-ring,wherein uncured potting material is prevented from leaking from thejoint between the potting cup and the base.